Arthroscopic impedance probe to detect cartilage degeneration

ABSTRACT

The change in tissue impedance due to the change in the extracellular matrix that results from the degradation of cartilage is utilized to detect degradation of articular cartilage. A probe comprising electrodes is applies a current to the articular cartilage which results in a current distribution and electric field within the cartilage, along with an associated voltage drop across the electrodes. The amplitude of this voltage drop is then measured and divided by the current applied to determine the tissue impedance. By measuring the impedance of patient tissue and comparing the detected patient impedance to a normal value for the tissue from clinically normal tissue, a determination of whether the patient tissue is degraded and the extent of degradation is possible. Preferably, the impedance is measured using a probe with interdigitated electrodes. By changing which electrodes are utilized, the wavelength of the current distribution changes, allowing the probe to image depth dependent focal lesions.

PRIORITY INFORMATION

[0001] This application claims priority from provisional applicationSer. No. 60/179,820 filed Feb. 2, 2000.

BACKGROUND OF THE INVENTION

[0002] The invention relates to the field of non-destructivearthroscopic diagnostic probes, and in particular to non-destructivearthroscopic diagnostic probes for detecting degeneration of articularcartilage utilizing impedance measurements.

Articular Cartilage

[0003] The function of organs in the human body are a direct consequenceof their inherent structure. The function of an organ as a whole is morethan the sum total of its individual constituents. Articular cartilage(AC) is a rich and illustrative example. An understanding of thecomposition and physical properties of AC are essential to diagnose adisease with any given device to aid in patient care. AC is a dynamic,living tissue that responds to stimuli in its environment (i.e. externalloading, fluid flow, electric fields), and the cells of cartilage(chondrocytes) are able to maintain its intricate extracellular matrix(ECM). The scientific data collected over the past 25 years for normalcartilage, supports a hypothesis that a feedback between mechanicalstimulation and chondrocytes must exist to maintain cartilagehomeostasis.

[0004] By gross visualization, during knee arthroscopy or an open jointprocedure, normal AC appears as a homogeneous shiny white substancecovering the ends of articulating bones. It is a thin layer from 1 mm to6 mm depending on the joint and particular surface location. In thepresence of synovial fluid, AC provides a very low friction surface thathas a coefficient of friction that is less than that of ice on ice. Acloser inspection at the light microscopic level reveals AC as a verycomplex ECM of macromolecules with chondrocytes embedded within.Cartilage is a unique organ as it is aneural, alymphatic and avascular.Nutrient exchange to the chondrocytes proceeds by diffusion fromsynovial fluid at the articular surface and from the subchondral bonebelow. The lack of a blood supply severely limits AC's ability to berepaired following injury. The absence of innervation means that pain istransduced from the surrounding bone through unshielded force, or fromthe joint capsule in response to an inflammatory stimuli response.

[0005] The complex structure of AC acts as a loadbearing, shockabsorbing, and wear resistant material to protect joint surfaces. Inaddition to a low friction surface, AC has a high compressive strengthcritical to pain free joint function. Compressive loads are distributedover a larger area, while also acting as a damping element during highimpact loading (i.e. jumping). Cartilage is also strong in tensileloading when subject to shear stresses due to the sliding nature ofjoint function (i.e. knee joint or intervertebral disk). Lubrication bythe synovial fluid also reduces shear stresses and helps protect thecartilage from trauma. These macroscopic mechanical properties are adirect consequence of the composition.

[0006] Articular cartilage is mostly water (60-80% of total weight) andECM that comprises the bulk of the dry weight. The primary structuralcomponents of articular cartilage (AC) ECM are produced and maintainedby the chondrocytes enmeshed within it. Tissue mechanical propertiesdepend on the organization and structure of macromolecules present inthe ECM. The ECM is made up of mainly collagen type II fibrils (alongwith small amounts of types IX and XI collagen), charged proteoglycans(PGs), and cells. Collagen and PGs form the framework for cartilage thatresists applied mechanical forces. The collagen forms a dense crosslinked network with PGs embedded within. Proteoglycans aremacromolecules that contain polyanionic sulfated glycosaminoglycan(sGAG) chains. The negative fixed charge density of sGAGs isapproximately 5.3 mEq/gm dry weight in normal human femoral headcartilage. A slight excess of mobile positive ions within the tissuepreserves electroneutrality. At the macroscopic level a Donnan osmoticswelling force develops, caused by the electrostatic charge repulsionbetween the fixed anionic groups that draws water into the ECM,expanding the collagen network.

[0007] The chondrocytes are responsible for PG turnover (synthesis anddegradation). The most abundant PG is aggrecan, which has an extendedprotein core with up to 150 chondroitin sulfate and keratan sulfatechains attached in a “bottle brush” structure providing a highconcentration of anions. When first synthesized, aggrecan is mobile, butquickly binds to immobile hyaluranon, stabilized by a link protein,creating the high density of fixed COO⁻ and SO₃ ⁻ groups at physiologicpH.

[0008] Many other soluble factors play an important role in themaintenance process by participating as mediators of turnover andproduction of ECM, including ions, growth factors, hormones, cytokines,proteinases (e.g. matrix metalloproteinases) and their inhibitors.Numerous factors are required to maintain homeostasis. They can beproduced by the chondrocytes themselves or synthesized elsewhere andtransported into the ECM. These factors affect the chondrocytes throughcell surface receptors and their transport through the ECM can beprohibited, resulting in pathology.

[0009] At the ends of articulating joints, the AC is 3-4 mm thick, withareas on the patella as high as 6-8 mm. Microscopically mature AC has 3zones based on the shape of the chondrocytes and distribution of thetype II collagen. The tangential layer has flat chondrocytes, tangentialcollagen fibril orientation and a sparse PG content. The intermediatelayer is the thickest, with round chondrocytes, oriented in verticalcolumns. Finally, the basal layer has round chondrocytes and containsthe tidemark that separates the uncalcified (nourished by the synovialfluid) and calcified cartilage (that gets fed by the episphysealvessels). It has been reported that no age changes after maturation arediscernible based on histology, including no loss of AC.

[0010] Collagen makes up the majority of the dry weight (approximately50%) of AC, as it is also the most common structural protein in thebody. In cartilage the most abundant form of collagen (>90%) is type IIthat acts as the structural meshwork of the ECM with its associatedextensive intermolecular crosslinking via trivalent hydroxylysylpyridinoline residues. The name “collagen” is a generic term forstructural molecules that are rich in glycine, proline andhydroxyproline. Striated fibrils, type I, II, and III have threepolypeptide chains wound in a triple helical configuration.

[0011] Type II collagen is composed of three left handed tightlyinterwoven alpha chains (α(II)₁), 300 nm long and 1.5 nm in diameter,each with a repeating amino acid sequence of GlyPro(Hydroxyproline). Itis the triple helical structure that enables collagen type II to have ahigh tensile strength. Hydroxylysine (some as hydroxylysyl pyridinolinecrosslinks) helps type II collagen link together the ECM network.

[0012] Small amounts of collagen type IX help connect the various matrixelements together while type XI (approximately 3%) regulates the caliberof the fiber. In addition, collagen types VI and X are also present(<1%). Collagen type VI has a cross linking behavior and an increasedamount has been reported in OA models, while collagen type X isassociated with growth plate cartilage in the hypertrophic zone, and inthe calcified layer of mature cartilage.

[0013] Individual aggrecan monomers are attached to a GAG core(hyaluronate), stabilized by link protein, with the number depending onthe functional nature of the cartilage. The common PGs that can be foundin cartilage include aggrecan, decorin, fibromodulin, and biglycan andmake up about 35% of dry weight of AC. Aggrecan molecules form largeaggregates (approximately 200 MDa) in cartilage, forming a hydrogel-likestructure that is, in turn, immersed within the collagen type II fibers.

[0014] Aggrecan is the major PG in AC (around 90%) and is composed oftwo types of sulfated GAG (sGAG): chondroitin-6-sulfate,chondroitin-4-sulfate (approximately 20,000 MW) and keratan sulfate(approximately 5,000 MW). The amount of sGAG attached to each PG variesdepending on the functionality and integrity of the tissue. Chondroitinsulfate GAGs are chains of repeating disaccharide units that containhighly charged carboxylate and sulfate groups. The high density ofnegatively fixed charge groups helps attract positive ions and create anosmotic swelling pressure to imbibe water within the tissue. There areapproximately 100-150 sGAG chains per aggrecan molecule, while extremelylarge aggregates can bind thousands of GAG chains.

[0015] At the molecular level, the main unit of the aggrecan molecule isa protein core of approximately 300,000 MW. It has 3 associated globulardomains: G1 and G2 at the N-terminus, and G3 at the C-terminus. The GAGsare mostly contained within the G2 to G3 domain. Keratan sulfate chainsbind, in general, closer to the G1-G2 region (interglobular domain). Theaggrecan PG varies in total size due to the varying amount of boundchondroitin sulfate. Therefore, proteoglycan is a combination of 5%protein and 95% carbohydrate.

[0016] Variation in the concentration of PGs has been observed withdepth from the articular surface in immature and mature tissue, as wellas location within a joint. Areas bearing higher stresses have also beenshown to have a higher PG content. The charge groups on the GAG chainsare ionized at physiologic pH providing a large osmotic swellingpressure that largely determines the equilibrium compressive modulus.

[0017] The main function of chondrocytes involves replenishment ofmacromolecular ECM constituents for its preservation in its harshmechanical environment. With respect to the volume of the cartilage, thechondrocytes account for less than 5% and have a density ofapproximately 20×10³ cells/mm³. In fully developed tissue the size,shape and density of the chondrocytes vary with depth proceeding downfrom the articular surface. In general, the size increases, the shapemoves from flat and elongated to spherical, and the density of cellsdecreases with increasing depth toward the underlying subchondral bone.

[0018] In order to perform their biosynthetic functions, chondrocytesare well equipped with an extensive endoplasmic reticulum and Golgiapparatus as well as mitochondria and secretory vacuoles. Chondrocytesare also involved in regulation of ECM assembly and repair by secretingand mediating their assembly. Because of its ability to producedegradating enzymes as well as their inhibitors, the chondrocytes arebelieved to participate in the physiologic as well as in the pathologicdegradation of ECM.

[0019] Chondrocytes have also been shown to be able to adapt to thechanges by biosynthetically responding to chemical, physical, mechanicaland electrical stimuli through their cell receptors. These adaptations,through balancing the homeostasis of the ECM, alter integrity to conformwith the stimuli. Biomechanical stimuli of cartilage explants (static orsmall amplitude dynamic compression) have been found to influence therate of aggrecan synthesis and catabolism. This behavior may be due tochanges in cell shape, specific cell-matrix interactions or change theavailability of growth factors.

[0020] Investigators are also now beginning to look past the cellmembrane to examine the intracellular changes due to mechanicalcompression on mRNA levels and also how cell deformation affectsintracellular organelles like Golgi apparatus or endoplasmic reticulumto fulfill their functions.

[0021] Water is the most abundant component in AC, and it appears to becompartmentalized. Water that exists in the interstices of collagenmolecules and fibrils is intrafibrillar water with the balance in theextrafibrillar space. The -distribution between these two compartmentshas been reported to be a function of the fixed charge density andloading configuration of the tissue.

[0022] The total amount of water present is dependent on the interactionbetween the collagen and sGAG components, as the collagen fibrilreinforcements in the tissue prevent full expansion by the sGAGs (due totheir fixed charge density) and thus constrain water intake. Thisbalance is perturbed in OA cartilage. An increase in water content isobserved compared to healthy cartilage, despite an observable reductionin GAG content. The explanation of this contradiction lies in theassumption that damage to the collagen network severely impairs itsability to restrain the sGAG swelling pressure (despite its lowerconcentration in the tissue), thus the amount of water increases. One ofthe hallmarks of pathologic cartilage is its increased water content.

[0023] Matrix metalloproteinases (MMP) are an important group of zinccontaining enzymes responsible for the breakdown of ECM components suchas collagen and PGs in normal embryogenesis and remodeling as well as inmany disease processes like cancer, osteoporosis and arthritis. Theseenzymes are almost universally distributed among mesenchymal cells ofall types, and in some epithelial and endothelial cells as well. Thefamily of MMPs can be divided into 3 subclasses: collagenases (MMP-1,MMP-8, and MMP-13), gelatinases (MMP-2 and MMP-9), and stromelysins.

[0024] The collagenases MMP-1 and MMP-13 are members of the family ofmatrix metalloproteinases that play an important role in the degradationand turnover of the ECM molecules such as type II collagen and aggrecanduring normal remodeling (e.g. embryogenesis) and disease processing.MMP-1 and MMP-13 have been implicated in the progression ofosteoarthritis and rheumatoid arthritis since they are found in theassociated tissues at higher levels than in normal human tissue. Theactive enzyme has also been found at the lesion sites on the tibialplateaus of Hartley guinea pigs during the progression of spontaneousosteoarthritis. MMP-1 cleaves the three chains of the type II collagenmolecule at specific sites, producing helical trimeric fragments of ¾(from the N-terminus) and ¼ in length that can be detected usingneoepitope antibodies. MMP-13 initially cleaves type II collagen at thissame site, but can additionally cause a second cleavage, residuescarboxy terminal to the primary cleavage site, and then a third cleavagesite another three residues carboxy terminal to the second cleavagesite. The time course of damage induced by MMP-13 is more rapid andtransient than that due to MMP-1; MMP-13 was also found to turn overtype II collagen 10 times faster than MMP-1 in humans.

[0025] It has been recently demonstrated through immunostaining ofneoepitopes in cartilage from patients with osteoarthritis that type IIcollagen degradation was initiated by MMP-1 and MMP-13 at the articularsurface and then extended into the middle and deep zones. In addition tothis depth dependence, a direct correlation between the Mankin score ofOA cartilage degradation and the intensity of the immunostaining of MMPswas recently found.

Electromechanical Effects of Articular Cartilage

[0026] As a consequence of the composition of cartilage, measurableelectromechanical properties are exhibited (an area of research referredto as cartilage electromechanics). To consider modeling and analysis ofAC behavior, AC can be conceptualized as a composite material of afibrous mesh (collagen) embedded in a highly hydrated charged gel ofPGs. Resistance to tension and shear loading is provided by thecollagen, whereas the high swelling pressure of the PGs enablescartilage to resist compressive loading. The engineering properties ofcartilage, i.e. compressive and tensile strength, have been assessed inmany different systems. Extraction of PGs has produced a marked decreasein the tissue's equilibrium compressive modulus without effects to thetensile stiffness, while selective degradation of the collagen networkpredominantly affects tensile properties of the tissue.

[0027] The dynamic behavior of cartilage is the result of interactionsbetween the solid ECM components and the interstitial fluid. Cartilageis most successfully modeled as a poroelastic medium. Such a medium is afluid saturated porous material in which viscous effects arepredominantly due to frictional interactions between the fluid and solidphases. The literature documents a rich history of mathematical modelsto describe this physical behavior of cartilage. Early work was begun byBiot almost fifty years ago in the context of geophysics. Using amixture theory, where material properties and constitutive relations arederived separately for the fluid and solid phases, a biphasic theorydescribing the behavior of cartilage was developed by Mow and coworkers.These models of fluid flow can be related to mechanical properties likestiffness. Interest today remains high, with continued efforts toproduce more complex nonlinear models for use with today's powerfulcomputational capacity to more realistically model high strain behaviorand the effects of nonhomogeneous material properties on experimentalmeasurements.

[0028] Besides having mechanical properties, cartilage exhibitselectrical properties that are coupled with mechanical stresses. It hasbeen previously shown that these electrical interactions play asignificant role in cartilage physiology.

[0029] The electromechanical transduction effect is a property of thecartilage composition, specifically the PGs and two dual phenomena areobserved.

[0030] The first of these effects is known as streaming potential. Asthe hydrated ECM of cartilage is mechanically deformed, a flow ofinterstitial fluid relative to the fixed charge groups of the solidmatrix is created. Entrained positive ions are separated from thenegatively charged matrix macromolecules, giving rise to a voltagegradient or streaming potential in the direction of fluid flow. Thesecond of these effects is the converse electrokinetic effect, known ascurrent generated stress. This effect results when application ofcurrent causes an electrophoretic motion of the negatively charged fixedECM molecules (PGs) towards the positive electrode and an electroosmoticmotion of the mobile ions of the fluid phase towards the negativeelectrode. These combined effects produce a measurable bulk mechanicalstress at the tissue surface that can be detected by an overlying stresssensor.

Osteoarthritis

[0031] Osteoarthritis (OA) is the most common disease that directlyaffects the everyday mobility and quality of life. It mainly strikes inthe last quarter of life, but also occurs in young people aftertraumatic sports injuries. It attacks the tissue of the synovial joints(e.g., knees, hips, and hands) and is characterized by pain andaccompanied by limitations in joint motion. It can progress to endstage,when patients can no longer walk pain free. Although the disease itselfis not a significant source of mortality, it is a great cause ofphysical suffering. In the US alone, estimates that the number of peoplesuffering from OA will reach 68 million by the year 2010.

[0032] Osteoarthritis describes a group of joint disorders that lead tothe destabilization of normal AC function. Degradation and synthesis bychondrocytes of ECM are uncoupled. The initiation of OA may be a resultof a variety of factors, including genetic, metabolic, developmental,and traumatic. OA is diagnosed clinically with sharp stabbing jointpain, tenderness, and inflammation leading to limitations of jointmovement.

[0033] One of the early events in OA at a molecular level is alterationof the cartilage ECM, and the loss of the highly charged macromolecules(PGs) from the matrix. These changes often occur in localized regions ofcartilage along the joint surface and to nonuniform depth. Investigatorshave hypothesized that such molecular changes should change the tissue'smaterial properties.

[0034] Traumatic injuries can cause focal defects in cartilage adjacentto otherwise normal cartilage. Clinical repair approaches includedebridement, microfracture, osteochondral plug resurfacing, andchondrocyte transplantation. During surgical procedures, and thesubsequent follow-up, surgeons need to assess the state andfunctionality of the repair tissue. Often remodeling leads to afibrocartilage repair tissue that appears cartilage like but has poorphysical properties that ultimately lead to its failure.

[0035] In addition, there is a great need for methods to assess theefficacy of therapeutic interventions developed to prevent cartilagedestruction or the patency of cartilage repair tissue. Presently, theassessment of cartilage repair is based on gross and microscopicmorphological features. Detailed studies have established that therepair tissue is generally of good quality in the short term, but failswith time. At present, this behavior is difficult to explain. Theliterature shows only a minimal molecular characterization of the typesof the cartilage repair tissue. The more that is understand of therepair process, the higher the chance to produce the optimal outcome; arepair tissue integrated in the native cartilage and biomechanicallyfunctional for many years. There is also a need for better in vivo,non-destructive diagnostic tools for quantitatively assessingdegenerative changes in articular cartilage and to diagnose OA.

Diagnosis and Monitoring of Osteoarthritis

[0036] Current diagnostic criteria and methods for monitoring OA arebased on external physical examination and x-rays. Efforts are beingmade to diagnose the disease progression at its earliest stages in orderto apply treatment before further damage can occur. Initial diagnosis ofOA begins with patients complaining of pain and stiffness in theirjoints. Further diagnosis can be made using the current gold standard ofx-ray radiography. A grading scheme to characterize the damage isutilized; the most commonly used scale of radiographic evidence is theKellgran and Lawrence method. The scale is numbered from 0 to 4 with 0being no visible defects and 4 showing visible OA.

[0037] Radiographic diagnostic criteria has three main shortcomings:they lack sensitivity, the emphasis is on changes to the bone, andreading the films is subjective with poor reproducibility. Studies haveshown that after 2 years of treatment with NSAIDS, the radiographsshowed no significant changes. It is possible that the sensitivity issimply too low to detect small treatment effects. The limitations ofradiographs include: nonstandard and shifting of joint positions, thex-ray beam alignment, radiographic magnification not taken into accountand landmarks for measurement can be subject to individualinterpretation. These defects have been corrected in current clinicaltrials. However, x-rays cannot visualize cartilage. They measure thespace between bony surfaces that could be filled with cartilage andthus, are an indirect measure of the absence of cartilage or a failureof compressive resistance of existing cartilage. Thus, x-rays onlyexamine cartilage status and can reveal bone and then joint involvementonly in the later stages of disease.

[0038] Laboratory methods such as a synovial fluid extraction or ahistological examination (Mankin scale) can be used to furtherinvestigate the progression of disease. Unfortunately, synovial fluidextraction can only rule out other possible causes for the pain, such asrheumatoid arthritis. The Mankin scale categorizes the extent of diseaseprogression in tissue via histology but requires a destructive biopsy.Furthermore, histological examination occurs only where tissue wasremoved.

[0039] Other possible methods include MRI, sonography, scintography, andbiochemical markers, but they too have limitations in detecting changesin cartilage. MRI concentrates on biochemical composition studies thatcould yield measures of the sGAG and collagen concentrations, but atthis stage the cost is very high and the fixed charge density too lowfor adequate resolution. As such, clinical MRI approaches do not havethe resolution to show early cartilage changes and do not measurephysical properties.

[0040] Arthroscopy has become an important technique in the diagnosisand therapy of knee OA. A 4 mm diameter arthroscope and/or surgicalinstruments along with a light source is inserted into the jointcapsule, allowing direct visualization of the cartilage surfaces,ligaments, and menisci. The complication rate and morbidity associatedwith the procedure are so low, that arthroscopy is increasingly beingperformed on joints that are only minimally symptomatic as anexploratory procedure. It can detect before degenerative changes areevident by radiography. The recent advent of a 1.8 mm diameter needlearthroscope is transforming arthroscopic examination from ahospital-based procedure to a routine office procedure. During a typicaldiagnostic arthroscopic examination of the knee, the orthopedic surgeoncan inspect the AC surface for gross changes. Arthroscopy is performedvisually without quantitative biophysical methods. Ultimately, thesegross visualization arthroscopic methods, give a visual picture of thecartilage that may or may not correlate with its physical properties.

[0041] It is important to note that early OA cartilage may appear normalby visual inspection. Given that arthroscopy is one of the most commonorthopedic procedures, visual inspection alone during arthroscopy maynot be sufficient for diagnostic purposes, indicating a need forquantitative approaches.

[0042] There is currently a commercial arthroscopy blunt probe tosubjectively assess the degree of softening (known as “grade Ichondromalacia”) that can result prior to x-ray changes. Another deviceand method to measure the physical properties of articular cartilage hasbeen developed by Dashefsky (Arthroscopy, 3:80-85, 1987). Dashefskydesigned a more objective measurement apparatus. He used an instrumentedindenter attached to a force transducer to qualitatively assess themechanical properties of chondromalacia of patellar cartilage duringarthroscopy. In a group of 107 knees with “patellofemoral symptoms andsigns”, 90% were evaluated as “soft;” but over half of these “soft”cartilages showed no detectable visual changes of the articular surfaceof the patella. Interestingly, of 58 patients with no signs or symptomsof the patella, 50% showed softening of the cartilage. These resultssuggest that physical property changes may not correlate with thepatients' symptoms until an irreversible threshold of damage occurs withthe chronic wear and tear of cartilage.

[0043] More recently, Lyyra et al. (Med. Eng. Phys., 17:395-399, 1995),developed an arthroscopic indenter instrument with strain gauges formeasurement of tissue stiffness in vivo, produced as Artscan 1000 fromArtscan Medical Innovations, Helsinki, Finland. A constant deformationis imposed on the cartilage by the indenter, and the “instantaneous”load response during a one second measurement interval is used toevaluate the tissue stiffness before appreciable stress relaxation hasoccurred. In order to compute an effective dynamic modulus, anindependent measurement of tissue thickness is necessary, as with anyindentation technique. The device was able to detect differences in thestiffness of cartilage in different regions of normal knees.Interestingly, however, the indenter detected only 30-40% decreases incartilage stiffness in the most severely affected regions of thepatellar cartilage of patients with known chondromalacia. Use of thesedevices indicates that purely mechanical tests alone (e.g., indentationtests) may not provide a sufficiently sensitive index of earlydegenerative changes in cartilage. Because of this, the cartilage'selectromechanical transduction properties have been incorporated into asurface diagnostic probe.

[0044] This approach has been termed electromechanical surfacespectroscopy and utilizes the current generated stress phenomena ofarticular cartilage. Spatial and temporal changes in the molecularintegrity of the collagen network due to degeneration lead to importantchanges in the functional mechanical and electrical properties of thetissue and therefore cause changes in the current generated stress. Inthis approach, illustrated in FIG. 1, small sinusoidal electricalcurrents are imposed by an interdigitated electrode array 104 that restson the cartilage articular surface 100. The current causes anelectrophoretic motion of the negatively charged cartilage extracellularmatrix (ECM) towards the positive electrode and an electroosmotic motionof intratissue fluid towards the negative electrode. These combinedeffects produce measurable normal mechanical stresses at the tissuesurface that can be detected by an overlying piezoelectric stress sensor102. The stress produced is at the same fundamental frequency as thedriving current, but out of phase due to the poroelastic nature of thecartilage response. The penetration depth into the tissue isproportional to the spatial wavelength of interdigitated electrode 104structure, defined as twice the electrode spacing.

[0045] This electromechanical technique is generally described in U.S.Pat. No. 5,246,013. In addition, for this technique, Frank et al. (J.Biomech., 20:629-639, 1987) utilized a uniaxial configuration in whichthe current was applied via electrodes on opposite ends of a excisedcartilage plug. Sachs et al. later completed a mathematical modelshowing that two silver electrodes placed on the same surface side ofcartilage could induce a measurable mechanical response when current isapplied in this potentially nondestructive arrangement (Physiochem.Hydrodyn., 11:585-614, 1989 and A Mathematical Model of anElectromechanically Coupled Poroelastic Medium. PhD thesis,Massachusetts Institute of Technology.). In parallel, Salant et al.(Surface Probe for Electrokinetic Detection of Cartilage Degeneration.MD thesis, Harvard-MIT Division of Health Sciences and Technology.) andlater Berkenblit et al. (Spatial Localization of Cartilage DegradationUsing Electromechanical Surface Spectroscopy With Variable Wavelengthand Frequency. PhD thesis, Massachusetts Institute of Technology.)improved on this design by designing a configuration in which currentcould be applied to a single surface of cartilage and the resultantinduced mechanical stress could be measured.

[0046] The general technique has also been termed “imposed-k sensing”because the medium is excited at a specified temporal (angular)frequency, by an electrode structure having a spatial period=2/kdetermined by the electrode geometry and hence a dominant wave number k.Its advantages are that it can be made nondestructively (an importantrequirement for in vivo measurement of cartilage properties) and theelectric fields generated decay exponentially into the material, with apenetration depth on the order of /5 to /3. Thus, different depths ofthe material may be tested by varying the imposed spatial wavelength,and spatial inhomogeneities in material properties can be detected bymaking surface measurements using a series of imposed spatialwavelengths (spatial localization). The depth to which the currentpenetrates into the medium is proportional to the effective spatialwavelength, which is equal to twice the center-to-center distancebetween adjacent electrodes. By changing the imposed spatial wavelength(by having independently addressable electrodes), various depths of themedium can be preferentially assessed.

[0047] The use of varying wavelengths is illustrated in FIGS. 2a and 2b, collectively. Both the frequency and wavelength of the appliedcurrent density affect the depth of penetration of the current inducedporoelastic deformation within the tissue. The characteristic depth ofpenetration of the current density, itself, is approximately the spatialwavelength of the current. This wavelength, is determined by theelectrode excitation pattern at the cartilage surface. Therefore, aprobe with four independently addressable electrodes 104 is utilizedsuch that connection to each electrode can be varied externally, therebyenabling multiple wavelengths to be applied using a single device.Applied current densities having short wavelength, illustrated in FIG.2a, compared to cartilage thickness are confined to the superficialregion of the tissue; the associated current generated stress willtherefore reflect the properties of the superficial zone. In contrast,long wavelength excitations, illustrated in FIG. 2b, penetrate the fulldepth of the tissue and thereby reflect the average properties of fullthickness cartilage. Thus, combinations of short and long wavelengthexcitations enable the probe to “image” depth dependent focal lesions.

[0048] Spectrometer 102 response depends in a sensitive manner onmolecular level changes in the cartilage matrix similar to changes thatoccur during the earliest phases of OA degeneration. These resultsprovide the fundamental basis for the in vivo surface electromechanicalspectroscopic approach to detect cartilage degeneration.

[0049] For this method, typically a sinusoidal current density of 1mA/cm² is applied to the tissue over the frequency range 0.025-1.0 Hzusing a bipolar operational amplifier, such that the total currentamplitude is constant at all frequencies, and driven by a programmablefrequency generator controlled through a computer. The output of thepiezoelectric sensor electrodes are passed through a high impedanceelectrometer, low pass filtered to remove 60 Hz noise and differentiallyamplified. The signals are recorded on a computer, and combined with amechanical sensor calibration done before each test to obtain thecurrent generated stress. By comparison of this current generated stressto the current generated stress in normal cartilage, a determination canbe made as to whether the present cartilage has experienced degradation.

[0050] While electromechanical spectroscopy is useful in detectingcartilage degeneration, there is still an ongoing need for in vivotechniques to non-destructively and rapidly test for cartilagedegeneration. Experiments have shown that use of the electromechanicalspectroscopy may be not be able to sensitively measure surface cartilagedamage, to a depth of approximately 50 μms, caused by MMP-13. Inaddition, the most sensitive detection afforded by current generatedstress techniques requires measurements at lower frequencies, whichrequire longer measurement time. The present invention overcomes thesedisadvantages, in addition to providing further advantages when utilizedalone, or in conjunction with electromechanical spectroscopy.

SUMMARY OF THE INVENTION

[0051] A probe for detecting the degeneration of mammalian tissue,particularly articular cartilage, which comprises a pair of electrodesfor applying a current to the mammalian tissue is provided. When currentis applied to the tissue there is a distribution of current density andan electric field within the tissue, and a resulting voltage differenceacross the electrodes applying the current. In the preferred embodiment,the voltage difference is measured by a computer, and the computernormalizes the voltage difference to the current applied to the tissue.This normalized parameter is defined as the tissue impedance. Theimpedance is indicative of the amount of degeneration tissue hasundergone.

[0052] In an alternative embodiment, electrodes of the probe comprise aninterdigitated array of electrodes allowing the wavelength, and hence,the depth of penetration of the current distribution into the tissue tobe changed. When a short wavelength mode is utilized, the impedancemeasurements are representative of a superficial layer's degeneration.When a long wavelength mode is utilized, the impedance measurements arerepresentative of the bulk tissue degeneration.

[0053] In an alternative embodiment, the probe additionally comprises astress sensor. The current applied to the tissue creates currentgenerated stress in the tissue. This stress is measured and additionallyused to determine whether degeneration has occurred. In one embodiment,impedance measurements are taken simultaneously with the currentgenerated stress measurements at the same frequency as the currentgenerated stress measurements. In a different embodiment, the impedancemeasurements are taken sequentially, i.e. after the current generatedstress measurements, at a frequency higher than those at which thecurrent generated stress measurements are made.

[0054] A method for detecting the degeneration of mammalian tissue,particularly cartilage, is provided. Current is applied to the tissuecreating a distribution of current density and an electric field withinthe tissue, and a resulting voltage difference across the electrodesapplying the current. The amplitude of the voltage difference across theelectrodes is measured and divided by the current applied by theelectrodes to give the tissue impedance. This tissue impedance iscompared to an impedance value of clinically normal tissue to determineif the tissue is degenerated.

[0055] In one embodiment, the voltage difference across the electrodesapplying current is measured simultaneously to a current generatedstress measurement and at the same frequency as the current generatedstress measurement. In another embodiment, the voltage difference ismeasured at a higher frequency than that used for current generatedstress measurements, either alone, or after current generated stressmeasurements have been made.

BRIEF DESCRIPTION OF THE DRAWINGS

[0056]FIG. 1 illustrates the sensor and electrodes for current generatedstress measurements.

[0057]FIG. 2 illustrates the short and long wavelength modes for currentgenerated stress measurements.

[0058]FIG. 3 illustrates a system according to the present invention formeasuring the impedance of cartilage to detect degradation of thecartilage.

[0059]FIG. 4 shows a schematic of a preferred embodiment of the currentsource.

[0060]FIG. 5 illustrates the general method for determining whethercartilage degradation has occurred in test cartilage utilizing impedancemeasurements.

[0061]FIGS. 6a and 6 b, collectively, illustrate the layers thatcomprise the electrode/transducer structure for the preferred embodimentof the probe.

[0062]FIGS. 7a and 7 b illustrate the electrode patterns for theexcitation electrodes and sensor electrodes of the preferred embodimentof the probe.

[0063]FIG. 8 illustrates the backing plate on the electrode/transducerstructure of the preferred embodiment of the probe.

[0064]FIGS. 9a-9 d, collectively, illustrate the probe structure for thepreferred embodiment of the probe.

DETAILED DESCRIPTION OF THE INVENTION Operation of the Invention

[0065] When current is applied to articular cartilage there is adistribution of current density and an electric field within the tissue,and also an associated voltage drop across the electrodes applying thecurrent. The amplitude of the measured voltage drop between electrodesdivided by the applied current amplitude is defined as the electricalimpedance. The electrical impedance of the tissue for frequency rangesat least up to approximately 1 kHz, and possibly further, is dominatedby the resistance of the hydrated ECM and this resistance is inverselyproportional to the density of mobile ions within the intratissue fluid.Thus, the electrical impedance of the tissue will increase withdecreasing PG fixed charge content or increased swelling at constant PGcontent, since both these conditions lead to a lower concentration ofmobile ions in the ECM by Donnan equilibrium. In addition, collagennetwork degradation will alter impedance.

[0066] This change in impedance due to the change in the cellular matrixthat results from the degradation of the cartilage is utilized to detectdegradation of articular cartilage. By measuring the impedance ofpatient tissue and comparing the detected patient impedance to a normalvalue for the tissue from clinically normal tissue, a determination ofwhether the patient tissue is degraded and the extent of degradation ispossible. Preferably, the impedance is measured using a probe withinterdigitated electrodes similar to that used for electromechanicalspectroscopy. In this manner, the probe is able to “image” depthdependent focal lesions.

[0067] The use of impedance to determine cartilage degradation providesfor a number of advantages:

[0068] Higher frequencies (e.g. 1 kHz) than those used forelectromechanical spectroscopy can be used, increasing the speed withwhich measurements are performed.

[0069] No need for complex signal processing.

[0070] For an impedance-only probe, there is no need for a mechanicalcalibration step and eliminates the difficulties of manufacturing probeswith the sensitive behavior of a piezoelectric sensor.

[0071] When impedance is utilized with current generated stress (CGS,i.e. electromechanical spectroscopy) measurements (in the low frequencyrange, 0.25-1.0 Hz, where CGS is detectable), impedance is measuredsimply by monitoring the voltage across the probe inputs.

[0072] Electrodes for impedance measurements are able to be madearbitrarily small in any arrangement to control spatial sensitivity ofthe impedance measurements to possible diagnose cartilage abnormalitiesin small animals (e.g. rats) where the cartilage may only be 100 μmsthick.

[0073] Acquisition of data over a wider frequency range.

[0074] An increased precision of measurements resulting from smallerstandard deviations.

[0075]FIG. 3 illustrates a system according to the present invention formeasuring the impedance of cartilage to detect degradation of thecartilage. A waveform generator 300 applies a sinusoidal voltagewaveform to a current source 304 such that the total current amplitudeis constant at all frequencies. A computer 302 controls waveformgenerator 300. The current generated by current source 304 is applied tothe electrodes 310, 312, 314 and 316. Preferably, multiple independentlyaddressable electrodes are utilized so as to be able to generate bothlong wavelength and short wavelength excitations. As previouslydescribed, applied current densities having short wavelength compared tocartilage thickness are confined to the superficial region of thetissue. Therefore, the associated measured impedance reflects theproperties of the superficial zone. In contrast, long wavelengthexcitations penetrate the full depth of the tissue and as such, themeasured impedance associated with long wavelength excitations reflectsthe average properties of full thickness cartilage. Thus, combinationsof short and long wavelength excitations enable the probe to “image”depth dependent focal lesions.

[0076] To measure the impedance, computer 302 measures the currentapplied to cartilage 108 through electrodes 310, 312, 314 and 316. Inaddition, computer 302 measures the voltage drop across the electrodesapplying the current to cartilage 308. By normalizing the voltage dropto the current applied, the impedance of cartilage 308 is measured. Theimpedance is then compared to clinically normal tissue to determinewhether degradation has occurred in the tested tissue. It should benoted that, while a computer is preferred to measure the voltage anddetermine the-impedance, other equivalent devices such as a voltmeter,an application specific integrated circuit, or a chip internal to theprobe can be utilized to either measure the voltage or determine theimpedance without departing from the scope of the present invention.

[0077] Detecting cartilage degradation using impedance relies ondetecting changes in tissue conductivity resulting from loss of aggrecancharge groups and/or degradation of the collagen network. However, evencomplete loss of GAG from the tissue would decrease the electricalconductivity by only about 28 %, while electrokinetic coupling, and thusthe current generated stress amplitude, would decrease to zero. Thus, asis the case with purely mechanical measurements of cartilage materialproperties, measures of purely electrical properties, such as impedance,may be a less sensitive indicator of degradative changes thanmeasurement of electromechanical quantities such as current generatedstress. Therefore, in the preferred embodiment, the impedancemeasurements are utilized in conjunction with the current generatedstress measurements previously described. When measured in conjunctionwith the current generated stress measurements, impedance measurementsare made in the low frequency range, 0.25-1.0 Hz, where currentgenerated stress is detectable, simultaneously with the currentgenerated stress measurement. However, the impedance measurements areable to be made alone, or following the current generated stressmeasurements. When the impedance measurements are made alone, orfollowing the current generated stress measurements, rather thansimultaneous with the current generated stress measurements, it ispreferable to make the impedance measurements in a frequency range ofapproximately 1 kHz in order to allow the measurements to be takenrapidly, making it less sensitive to surgeon. hand tremor. In addition,measurement of cartilage impedance over a range of frequencies couldalso provide additional information about the tissue, such as anestimate of cartilage thickness.

[0078]FIG. 4 shows a schematic of a preferred embodiment of currentsource 104 that delivers current to the probe at the cartilage surface.Current source 304 is depicted used with a two-electrode probe forsimplicity.

[0079] Current source 304 comprises a bipolar operational amplifier 400,resistor 402, variable resistor 404 and resistor 406. When a sinusoidalinput voltage (V_(in)) is provided to current source 304 from waveformgenerator 300, a resultant current I_(c) to the cartilage is produced.The current applied to cartilage also results in a voltage differenceV_(out)-V_(m), between the electrodes on cartilage 408. This voltagedifference is measured and divided by the current IC to obtain themeasured impedance Z_(meas).

[0080]FIG. 5 illustrates the general technique for determining whethercartilage degradation has occurred in test cartilage utilizing impedancemeasurements. A sinusoidal current is applied to the electrodes on thesurface of the cartilage by a current source 500. As the current isapplied to the cartilage, the voltage difference present across theelectrodes is measured 502 by a computer. The measured voltagedifference is then divided by the current applied to the electrodes toobtain the impedance 504. This impedance is then compared to theimpedance of clinically normal cartilage to determine if degradation ispresent 506.

Exemplary Embodiment of Arthroscopic Probe for Detecting CartilageDegradation

[0081] Electrode/Transducer Structure and Fabrication

[0082] It should be noted, as the preferred embodiment of the presentinvention comprises impedance measurements utilized along with currentgenerated stress measurements, the preferred embodiment of theelectrodes and probe include a stress sensor and the followingdescription will include the stress sensor as part of the electrodes andprobe. However, the present invention should not be seen as limitedthereto.

[0083] The electrode/transducer structure (ETS) is the working componentof the “probe” system, applying current to the cartilage surface andmeasuring the resultant stress from a piezoelectric film. Preferably, itis a flexible, 180 μm thick, 3 layer laminated structure. FIGS. 6a and 6b, collectively, illustrate the layers it is comprised of:

[0084] Silver excitation electrodes are etched from a sheet of silverfoil 604 and deposited with a layer of silver chloride in an appropriateelectrochemical cell. Preferably, silver foil layer 604 for theexcitation electrodes is made from a 25.4 μm thick silver foil cut to asize of 18×18 mm with a sharp straight edge. The use of Ag/AgClelectrodes decreases the low frequency impedance between the electrodeand cartilage while stabilizing the electrode potential.

[0085] A shielding layer 602 is preferably made from 25.4 μm thickMylar™ polyester film metallized (on one surface) with a thin layer ofaluminum and is cut to 15×15 mm. Shielding layer 602 separates thesilver electrodes and stress sensors while also serving as aground/isolation plane. The metallization is a ground plane that helpsshield the sensitive stress sensors from electromagnetic interference(principally electric fields emanating from the excitation electrodes)when the ETS is placed in the probe body.

[0086] The stress sensor is fabricated from a single sheet of 52 μmthick polyvinylidene fluoride (PVDF) with a thin (100 Å) sputterednickel-copper alloy metallization on both sides 600, known as Kynar™,from AMP Inc., Norristown, Pa. Preferably, Kynar™ layer 600 is punchedto a disk of 4.5 mm in diameter (the size of the “active” area of theprobe). This piezoelectric material transduces the mechanical stresssensed to a measurable voltage signal. The metallization on one surfaceof the Kynar™ is etched to form electrodes that register with the silverelectrodes on the opposite side of the ETS. The other side of the PVDFis connected to the ground/isolation layer 602. Kynar is preferable forthe stress sensor in this application for a number of reasons:

[0087] it has a high sensitivity to mechanical stress especially in thislow frequency (0.025-1.0 Hz) application, allowing the film to behave asa compact strain gauge with no external power source,

[0088] the generated dynamic signals are greater than those from typicalstrain gauges after amplification, and

[0089] the flexible sheets are relatively inexpensive and can be cutinto arbitrary patterns.

[0090] To characterize the sensor as a stress gauge, a force appliednormal (forces in the plane of the electrode are neglected) to thesurface of the film develops an electric surface charge on themetallization proportional to the mechanical stress. The charge Q[coulombs] developed by a stress [N/m²] over an area A [m²] can bedescribed by Q=d_(t)A, where d_(t) is an empirically determinedpiezoelectric strain constant. The open circuit voltage between themetallization on either side of sheet 600 is the charge divided by thecapacitance, A′/, where is the dielectric constant of the film and isthe film thickness. If part of the total metallized area is not beingloaded, it adds to the capacitance without generating any charge, thusdecreasing the measured voltage. Representing the total area by A′ andthe active area being loaded by A, the equation for the open circuitvoltage signal becomes:

V=Q/C_(total)=d_(t)A/A′/=d_(t)A/A′

[0091] For maximum sensitivity, it is desirable to maximize the measuredvoltage signal, V, for a given stress, thus A/A′˜1. The voltage outputalso depends on film thickness. As electrodes are more tightly packedonto the ETS, a thinner film may be needed to gain sufficient spatialresolution with respect to electrode spacing.

[0092] The assembly procedure of fabricating this laminated structure ischaracterized in the following phases:

[0093] Phase I-ETS Construction

[0094] The sheets are rinsed with a mild detergent and then deionizedwater, while handling is done with disposable latex gloves to keep allmaterials clean to prevent contamination with oils and dirt. To aid inthe photofabrication, both sides of the silver foil are gently abradedwith a fine abrasive and then dipped in a 15% v/v nitric acid solution.To form a laminated structure, silver foil 604 is bonded to thenon-metallized side of the shielding layer 602 using a two-part urethaneepoxy in a 50:1 ratio (e.g. Tycel 7000/7200, Lord Corp., Erie, Pa.)thinned with methyl ethyl ketone. Silver foil 604 is larger to allowpress fit connections with copper tabs in the periphery of the innercore when the probe is assembled. Sensor layer 600 is bonded to themetallized side of the shielding layer 602 with a manually applied thinfilm of silver conducting epoxy (e.g. TRA-DUCT 2902, TRA-CON Inc.,Medford Ma.). The ETS is pressed together for a few minutes to assuregood bonding and is allowed to cure overnight.

[0095] Phase II-Photofabrication

[0096] To form the silver excitation and the piezoelectric stress sensorelectrodes, standard photofabrication techniques are used. The surfacesare coated with a light sensitive organic polymer, photoresist, whichbecomes inert to the etching chemicals when cross linked by ultravioletlight. A negative of the electrode pattern desired is used toselectively cross link the photoresist, then etching chemicals are usedto isolate the electrodes.

[0097] The ETS is dehydration baked at 80° C. for 10 minutes in aconvection oven to remove residual moisture, and both sides are coatedwith a photoresist compound, hung to dry for 30 minutes in a darkroom,and then baked between paper and glass plates (to keep them flat) at 80°C. for another 10 minutes. Electrode patterns are converted to negativeimages (masks) on two photographic transparencies. A dry, photoresistcoated ETS is placed between the two masks, aligned so the electrodesare registered on opposite sides of the ETS, then exposed to ultravioletlight for 15 minutes. The ETS is then bathed in a xylene-based developersolution for 30 seconds, transferred to another bath of developer for 30seconds, and then rinsed under warm tap water and blotted dry. Thedeveloper removes the uncross linked photoresist, leaving the resistbehind in the desired electrode pattern. The electrode pattern for thesensor electrodes on Kynar™ layer 600 is illustrated in FIG. 7a, whilethe excitation electrode pattern for silver layer 604 is illustrated inFIG. 7b.

[0098] Phase III-Etching

[0099] Etching of the silver metallization occurs while the ETS ismounted in a custom made two-part poly(methyl methacrylate) holder witha rubber O-ring gasket to contain the etchant. An appropriate etchant isa 55% w/v solution of ferric nitrate heated to 45° C., with the silverside of the ETS exposed to a fresh etchant bath every two minutes. Thethin metallization on the piezo film is etched, by carefully placing afew drops of etchant on the surface, waiting only 5-10 seconds, andquickly rinsing with deionized water. The photoresist is finally removedfrom both sides with a cotton swab dipped in xylenes.

[0100] The next step in ETS fabrication involves cutting the ETS into apattern that enables it to be fit onto the head of the probe. During thephotofabrication step, the outlines of the border were also marked ontothe ETS. Cutting is performed along the borders with a sharp scalpel. Asillustrated in FIG. 8, after cutting, a 0.33 mm thick crucifix shapedplastic backing plate 800 is attached to the piezo side of ETS 802 by atwo-part epoxy and dried overnight. Backing plate 802 helps with thealignment of the ETS, described below, and ensures ETS is flat. Thepresent design consisting of the backing plate and the smaller Kynardisk means that the ETS does not have to formed into its final threedimensional shape with a die. The final active area of the ETS is aflat, wrinkle-free surface against the top surface of the core, with nosmall fold or wrinkle introducing large local stress concentrations thatare sensed by the piezo film, significantly distorting the measuredsignal.

[0101] As a final step, a layer of silver chloride is layered onto thesilver excitation electrodes. The fully assembled probe is suspended ina bath of unbuffered 0.1 M NaCl, titrated to pH 4.0 with 1 N HCl, andthe positive terminal of a variable DC power supply connected to one ofthe silver electrode wires, in series with an ammeter and a 47 kWresistor. The negative terminal is connected to a platinum strip andsuspended in the electrolyte. A current of 120 μA is run for 10 minutes,corresponding to a total chloride deposition of 1000 (mA-seconds)/cm²,for each electrode of 1.59 mm² which is acceptable for bioelectricapplications.

[0102] Probe Structure and Assembly

[0103]FIGS. 9a-9 d illustrate, collectively, the structure of the probe.Generally, ETS 912 is held in place by pressing it against the inside ofa sheath 916 by an inner core 902. This assembly is held inside a shell918 with a screwed pusher/plunger 906.

[0104] The first part of the inner core 902 is preferably a stainlesssteel head, to accept the stress sensor contacts 910 and is conductingto provide part of the required shielding. Contacts to the stress sensoron ETS 912 are formed by metallic rods 910, preferably brass, at 90°intervals, potted into a recess in the stainless head with anon-conducting two-part epoxy 908. A cruciform pattern is machined inthe hardened epoxy 908 to accept a backing plate 800 constructed on ETS912, illustrated in FIG. 9d. Contacts 910 are electrically isolated fromeach other and the ground plane (stainless steel head). Prior topotting, each of the 4 contacts 910 are carefully soldered to a wire acable, which leads to the peripheral circuitry. The cable accommodatesfour coaxial cables that are especially tailored for low noiseapplications. The second part of inner core 904 is a plastic body, thatprovides a means of making the electrical connections to the excitationelectrodes, through 4 slots on the periphery that each hold a copper tab914 connected to a thin wire to carry the driving current.

[0105] The insulating (non-conducting) sheath 916 is a thin cylindricalshell made of plastic. Sheath 916 is fitted over ETS 912 that is placedover the end of the inner core 914. The end of sheath 916 is open,exposing the surface of ETS 912 to the cartilage during measurement.Sheath 912 is long enough to cover copper tabs 914 to prevent itscontact with stainless steel outer body 918, thus isolating the drivingcurrent from ground. In addition, the edge of the sheath is angled topress fit ETS 912 over the rim of the stainless steel head 902 of theinner core 900. The contacts between the silver electrode arms of ETS912 and copper tabs 914 of inner core 900 are also stabilized by thesheath 916 and sheath 916 provides the pressure to make contact betweenthe electrode arms of ETS 912 and copper tabs 914. A bead of siliconadhesive to seal the probe against the aqueous environment is placedaround the periphery of the angled edge before final assembly.

[0106] The outer body stainless steel tube 918 is a cylindricalstainless steel tube that acts as a stiff cover to protect the innercomponents of the probe. One end of outer body 918 is open to expose thesurface of ETS 912 but angled to catch the edge on the end of the probe.The other end of outer body 918 is flared outward. Outer body 918 isslid over the sheath/inner core 916. A nut is then slipped over outerbody 918, making contact with the threads on pusher/plunger 906, whichholds the inner core and non-conducting sheath assembly in outer shell918. The nut also pulls down upon the flared end of the outer body. Asthe nut is screwed, outer body 918 is tightened over sheath/inner core916.

[0107] As previously described, the head of the probe has a machinedrecess 920 in the shape of the crucifix so that ETS 912 fits with theproper orientation to line up the electrical contacts. Brass contacts910 at the head of the probe receive signals from the piezo electrodeswhile copper tabs 914 on the side inner core 900 connect with the armsof the silver electrodes. The current is driven through wires leading upto copper tabs 914 and onto the silver electrodes while the currentgenerated stress is transferred to the piezo electrodes and transmittedthrough brass contacts 910 to the output wires. When assembling theprobe, once contacts have been made and pusher/plunger 906 is fittedwith inner core 900, sheath 916 is fitted over the head of the probe,making sure ETS 912 is lying flat on the surface of the head. Beforesheath 916 is completely fitted over the head, the silver leads areslipped under the copper tabs, and the electrical connections to thesilver electrodes are tested. Once all contacts are established, a thinlayer of waterproof silicon rubber adhesive is placed on the inside edgeof sheath 916. Finally the outer stainless steel cylindrical body 918 isslipped over the probe. Adhesive is also placed on the inside edge ofouter body 918 before the probe is finally assembled. Excess adhesive iswiped off the edges. The sealed probe is allowed to dry for at least 24hours. The active surface of ETS 912 extends slightly beyond the tubeend and makes unobstructed contact with the cartilage surface.

[0108] Although the present invention has been shown and described withrespect to several preferred embodiments thereof, various changes,omissions and additions to the form and detail thereof, may be madetherein, without departing from the spirit and scope of the invention.

What is claimed is:
 1. A method for detecting degeneration in mammaliancartilage comprising: applying a current to said cartilage between atleast two surface points of said cartilage; measuring an amplitude of avoltage difference between said at least two surface points resultingfrom said application of current to said cartilage; dividing saidmeasured amplitude of said voltage difference by an amplitude of saidcurrent to determine an impedance of said cartilage, and comparing saidimpedance to a normal impedance value of said cartilage previouslydetermined from clinically normal cartilage to detect degeneration ofsaid cartilage.
 2. The method for detecting degeneration in mammaliancartilage, as per claim 1, wherein said current is applied into saidcartilage at varying depths by changing a spatial distance between saidat least two surface points to vary a spatial wavelength of saidcurrent.
 3. The method for detecting degeneration in mammaliancartilage, as per claim 2, wherein said current is applied to saidcartilage via electrodes of an interdigitated electrode array and saidspatial distance between said at least two surface points is varied byselectably addressing different electrodes of said interdigitatedelectrode array to apply said current.
 4. The method for detectingdegeneration in mammalian cartilage, as per claim 2, wherein saidcurrent is applied into said cartilage a short distance from a surfaceof said cartilage compared to an overall depth of said cartilage todetermine an impedance of a superficial region of said cartilage.
 5. Themethod for detecting degeneration in mammalian cartilage, as per claim2, wherein said current is applied into said cartilage substantially thefull depth of said cartilage to determine an average impedance of saidcartilage.
 6. The method for detecting degeneration in mammaliancartilage, as per claim 1, wherein said measuring step is performedsimultaneously with a current generated stress measurement.
 7. Themethod for detecting degeneration in mammalian cartilage, as per claim6, wherein said current is applied at a frequency in the range of0.025-1 Hz.
 8. The method for detecting degeneration in mammaliancartilage, as per claim 7 wherein said current is sequentially appliedat a plurality of frequencies in the range of 0.025-1 Hz and saidmeasuring step is performed at each of said plurality of frequencies. 9.The method for detecting degeneration in mammalian cartilage, as perclaim 1, wherein said current is applied at a frequency of about 1 KHz.10. A system for detecting degeneration in mammalian cartilage bydetermining an impedance of said cartilage, said system comprising: aself contained surface probe having a set of excitation electrodes forapplying a current to said cartilage between at least two surface pointsof said cartilage; a current source operatively connected to saidelectrodes and providing said current to said electrodes, and acomputing device operatively connected to said electrodes for measuringan amplitude of a voltage difference between said excitation electrodesresulting from said application of said current and normalizing saidamplitude to said current to determine said impedance of said cartilage.11. The system for detecting degeneration in mammalian cartilage, as perclaim 10, wherein said set of excitation electrodes comprises aplurality of selectably addressable interdigitated electrodes.
 12. Thesystem for detecting degeneration in mammalian cartilage, as per claim10, wherein said excitation electrodes comprise a metal salt.
 13. Thesystem for detecting degeneration in mammalian cartilage, as per claim12 wherein said excitation electrodes comprise silver chloride.
 14. Thesystem for detecting degeneration in mammalian cartilage, as per claim10, said self contained surface probe further having a stress sensor fordetecting a current generated stress induced in said cartilage by saidapplied current and transducing said current generated stress into oneof a voltage or current, said stress sensor comprising a set ofelectrodes for transmitting said voltage or current to said computingdevice, said computing device performing processing on said receivedvoltage or current to determine an electrokinetic parameter of saidcartilage.
 15. The system for detecting degeneration in mammaliancartilage, as per claim 10, said system further comprising: a waveformgenerator, said waveform generator operatively connected to said currentsource and providing said current source with a time varying voltagewaveform, said time varying voltage waveform causing said current sourceto generate a time varying current for application to said cartilage viasaid excitation electrodes.
 16. The system for detecting degeneration inmammalian cartilage, as per claim 15, wherein said current sourcecomprises: a bidirectional op-amp, said op-amp having an inverting inputconnected to ground and an output connected to a first excitationelectrode of said set of excitation electrodes; a first resistor havinga first terminal operatively connected to an output of said waveformgenerator and a second terminal connected to a non-inverting input ofsaid op-amp; a variable resistor having a first terminal connected tosaid non-inverting input of said op-amp and a second terminal connectedto a first circuit node; a second resistor having a first terminalconnected to said first circuit node and a second terminal connected toground, and a third resistor having a first terminal connected to saidfirst circuit node and a second terminal connected to a secondexcitation electrode of said set of excitation electrodes.
 17. Anarthroscopic probe for applying a current to mammalian cartilage inorder to determine an impedance of said mammalian cartilage so as todetect degeneration of said cartilage, said probe comprising: an innercore, said inner core having a first surface, a second surfacesubstantially parallel to said first surface and at least one sidesurface connecting said first and second surface; said first surfacehaving a set of electrodes mounted thereon, said electrodes having atleast two electrical conductors extending from said set of electrodesalong said at least one side surface; said at least one side surfacehaving at least two electrically conductive plates thereon, saidelectrically conductive plates for respectively contacting said at leasttwo conductors extending along said side surface, each of said at leasttwo electrically conductive plates having electrical conductorsconnected thereto for providing said current to said electrodes, saidelectrical conductors connected to said at least two electricallyconductive plates extending through a substantially hollow portion ofsaid inner core; said second surface having a recessed portion, saidrecessed portion connecting to said substantially hollow portion of saidinner core; a hollow pusher/plunger having one end thereof engaging saidrecess formed on said second surface, said electrical conductorsconnected to said conductive plates extending from said recessed portioninto said hollow pusher/plunger and extending inside said hollowpusher/plunger; a first sheath covering at least a portion of said innercore, said first sheath having an opening at an end co-located to saidfirst surface for exposing said set of electrodes, and a second sheathcovering said first sheath, said inner core and at least covering aportion of said hollow pusher/plunger, said second sheath having anopening at an end co-located to said first surface for exposing said setof electrodes.
 18. The arthroscopic probe for applying a current tomammalian cartilage in order to determine an impedance of said mammaliancartilage so as to detect degeneration of said cartilage, as per claim17, wherein a first portion of said inner core, said hollowpusher/plunger and said second sheath comprise a conductive material,and a second portion of said inner core and said first sheath comprise anon-conductive material.
 19. The arthroscopic probe for applying acurrent to mammalian cartilage in order to determine an impedance ofsaid mammalian cartilage so as to detect degeneration of said cartilage,as per claim 17, wherein said first sheath provides pressure to said atleast two electrical conductors extending from said set of electrodessuch that said at least two electrical conductors contacts saidelectrically conductive plates.
 20. The arthroscopic probe for applyinga current to mammalian cartilage in order to determine an impedance ofsaid mammalian cartilage so as to detect degeneration of said cartilage,as per claim 17, wherein said set of electrodes comprises at least fourinterdigitated electrodes, said at least two electrical conductorsextending from said electrodes comprises at least four electricalconductors and said at least two electrically conductive platescomprises at least four electrically conductive plates.
 21. Thearthroscopic probe for applying a current to mammalian cartilage inorder to determine an impedance of said mammalian cartilage so as todetect degeneration of said cartilage, as per claim 17, wherein saidfirst surface comprises a shaped recess formed thereon and said set ofelectrodes comprises a correspondingly shaped backing plate formedthereon, said shaped recess and said shaped backing plate providing aproper orientation such that said electrical conductors extending fromsaid set of electrodes are aligned with said electrically conductiveplates when said set of electrodes is mounted on said first surface. 22.The arthroscopic probe for applying a current to mammalian cartilage inorder to determine an impedance of said mammalian cartilage so as todetect degeneration of said cartilage, as per claim 17, wherein said setof electrodes is a combined electrode and transducer structure.
 23. Thearthroscopic probe for applying a current to mammalian cartilage inorder to determine an impedance of said mammalian cartilage so as todetect degeneration of said cartilage, as per claim 22, wherein saidcombined electrode and transducer structure comprises: interdigitatedexcitation electrodes formed of a conductive material for application ofsaid current to said mammalian cartilage; an insulating sheet havingsaid excitation electrodes bonded to the lower insulating surfacethereof and having its upper surface metalized for electrical grounding,and a piezoelectric polymeric film for transducing mechanical stress toone of a current or a voltage having its upper and lower surfacesmetalized, said lower metalized surface of said piezoelectric polymericfilm bonded to said upper metalized surface of said insulating sheet,said upper metalized surface of said piezoelectric polymeric film formedinto transducer electrodes for transmitting said current or voltage to adetector.
 24. The arthroscopic probe for applying a current to mammaliancartilage in order to determine an impedance of said mammalian cartilageso as to detect degeneration of said cartilage, as per claim 22, whereinsaid probe is additionally utilized to make a current generated stressmeasurement of said mammalian cartilage.